The present invention relates to a method of extracting dust that is separated on creping a soft crepe paper web off a Yankee dryer by means of a creping doctor, where part of the dust is carried along the web in a boundary layer of air which, because of the movement of the web from the Yankee dryer towards a reel, becomes entrained and follows the web. The invention also relates to an apparatus for carrying out the method.
When creping a soft crepe paper web, for example a tissue web, off a Yankee dryer, dust in the form of fibres and other particles is separated from the web. In order to lead that dust away, it has been proposed in U.S. Pat. No. 4,019,953 to Nystrom that a collecting receptacle should be mounted under the dust generating zone. To this receptacle, a compressed-air line and a suction line are connected for carrying away dust laden air entrained by compressed-air jets across the path of movement of the web. However, a flow of compressed-air is necessarily directed against the web, since to just suck away the dust through the ventilation has a negligible effect at this distance.
Also, such an apparatus is unwieldy, requires a lot of space, is relatively inefficient, and it only takes care of that part of the dust that falls down under the dust generating zone. The high velocities of modern tissue machines contribute to the poor result, i.e. in the range of 25 m/s, which means that dust laden air is entrained on both sides of the travelling tissue web, which is very fragile because of its low grammage. The apparatus proposed in U.S. Pat. No. 4,019,953 is not used in actual operation.
The separation of dust from the surface of a paper web is a problem also at slitting, slitting-winding and rewinding of the paper web. U.S. Pat. No. 3,775,806 to Olbrant, et al. proposes an arrangement for vacuum-cleaning the surface of a paper web. In order to avoid that the paper web during the vacuum-cleaning is sucked against the suction device and thereby damaged, a flow of air is directed towards the web simultaneously with the vacuum suction. The proposed vacuum-cleaning device thus has an upward open, horizontal sheet steel channel surrounding a square blowing-air pipe and extending across the web. The side of the blowing-air pipe facing the web has eyelid type openings turned from each other, to direct compressed-air jets principally parallel to the web both against and along the path of transportation of the web and thereby attaining a fixed distance of the web from the vacuum-cleaning device. Intermediate openings of the same type can be provided for directing compressed-air jets towards the surface of the web, to improve the effect of the vacuum cleaning by blowing dust off the surface. The blown air and released dust are sucked away in a direction perpendicular to the web through the two gaps that are formed upstream and downstream of the blowing-air pipe between the same and the surrounding sheet steel channel, to which an air sucking means is connected. The wall parts of the sheet steel channel, which are situated nearest the web, may be vertically adjustable in such a way that the blown-in, dust conveying air does not flow past the suction gaps and that the paper web does not scrape against the wall parts and become damaged.
The vacuum cleaning effect that is achieved by the arrangement according to U.S. Pat. No. 3,775,806 does not, though, meet today's demands when it comes to also taking care of the dust that follows in the air around a travelling paper web. Furthermore, this arrangement becomes both space demanding and expensive to buy and operate because of the compressed-air flow needed by the blowing-in technology.
The object of the present invention is to achieve an improved catching and extraction of the dust that results from the creping of a tissue web or any other soft crepe paper web and to thereby achieve an improved work environment for the operating personnel involved.
According to the invention, this object is achieved in the above stated method by mounting a dust extractor having an internal space and a web stabilizing imperforate plate in the immediate vicinity of an intended path of travel of the creped web so that the web during the travelling will take a fixed position, close to the imperforate plate, and eliminating at least an essential part of the dust-containing boundary layer of air by mere vacuum suction of the internal space.
In an apparatus for carrying out the method, the aforementioned object is achieved in a corresponding way, according to the invention, in that the apparatus comprises a hood of paper web width, having a front side edge and a rear side edge in relation to the advancing direction of the web, an imperforate plane cover sheet located to substantially close the hood while defining an internal space and forming a gap between at least one of said side edges and the cover plate, means for producing a subatmospheric pressure in the internal space, so that air is sucked through the gap, said apparatus being adapted to be mounted with its cover plate in the immediate vicinity of an intended path of transportation of the creped web, whereby the plane imperforate cover plate has a web stabilizing effect, so that the web at transport will take a fixed position close to the cover plate and the gap will be situated in the dust-containing boundary layer.
The invention is based on the realization that in order to solve the dust problems in tissue machines and the like it is necessary to suck away the dust laden boundary layers of air, which on creping, build up around the fast advancing web and to manage to suck sufficiently close to the creped web, the web must be stabilized and prevented from fluttering and the position of the web must be fixed. By the use of an imperforate plane, the web will be stabilized and fixed at a comparatively very short distance from the plane, without simultaneously getting a detrimental slowing down of the web against the plane, and hereby it is possible to work with vacuum suction only, instead of combined blowing and sucking to eliminate the dust. Advantageously, the distance is so short that the web very lightly touches the plane.
When vacuum sucking at a front or upstream edge of the apparatus in relation to the direction of web travel, it is suitable that the air flow just before and in the suction gap is mainly parallel to the web plane, so as to minimize such detrimental whirling that could convey part of the dust laden boundary layer away from the dust extracting apparatus. At the rear or downstream edge, it is, however, suitable that the air just before getting sucked into the suction gap also has a component of movement directed towards the web, so as to entrain dust that is very close to or on the web surface. Preferably, the suction velocity is of the same order of magnitude as the travelling velocity of the web. Thereby, at said rear edge, the relative velocity between the air stream at the suction gap and the web will be in the range twice the web velocity, which will give a really good dust extraction effect.
At creping, most of the dust arises on the side of the web that comes into contact with the creping doctor, but since the boundary layers on both sides of the web will contain dust, it is of course suitable to extract by sucking only an essential portion of each of the two boundary layers.
Part of the dust that is separated from the web at the doctor blade, which usually is located under the web and under the point where the web is doctored off the Yankee dryer, is not caught and conveyed off by the boundary layer on the underside of the web but falls downwards. To catch and extract even that falling dust, it is advisable to provide a dust suction box adjacent to the doctor beam. The suction box has a cover, which is sloping downwards and outwards from the Yankee dryer to prevent the web from getting stuck to the cover in the event of a possible web break.
At a certain distance downstream of the dust suction box, there suitably is a dust extractor working on suction basis only, to suck off the boundary layer of the web underside. Preferably, the distance will be such that under normal operating conditions, i.e. at normal web velocity and with normal suction forces in the dust suction box and in the dust extractor, between these two suction devices, an upwards directed air stream is created, which essentially extends over the whole mentioned distance and is strong enough to carry substantially all of the dust particles separated from the web.
Since the creping doctor blade is not in close contact with the Yankee dryer, part of the dust particles will pass inbetween the blade and the roll. To take care of them, it is suitable to provide a second dust suction box on the opposite side of the doctor beam in relation to the first dust suction box. The second dust suction box can be designed as the first one but reversed.
Preferably, the dust extractor is mounted in the immediate vicinity of the intended path of travel of the creped web in a position such that the web will touch the imperforate plane. On a macroscopic scale, this touch is defined as at least a line of contact extending across substantially the whole width of the web. Such a line of contact is suitably located at least at a downstream end of the imperforate plane but can, in addition, be located also at an upstream end of the plane. The touch of the web against the plane gives a certain working of the creped paper web, which effectively contributes to the extraction of web carried dust by the suction at the downstream end of the extractor. There, the web will be subjected to a pulse of air that passes through the creped porous web from its underside to its upperside when the extractor is placed on the upperside of the web. On passing through the web, the air pulse will dislodge dust particles, so that they may be carried away by the air flow sucked into the downstream end of the extractor. A certain dust particle dislodging effect may also result from microturbulence caused by shear in the thin boundary layer of air between the web and the dust extractor.
The invention gives a very efficient catching and extraction of the dust that is separated from the web at creping and it, thus, brings about a decided and very marked improvement of the work environment for the operating personnel concerned.
Further features of the invention and what is achieved thereby will be apparent from the following description. The invention will hereafter be described more in detail with reference to the enclosed drawings.